Conductive paste, method for forming an interconnection and electrical device

ABSTRACT

According to embodiments of the present invention, a conductive paste is provided. The conductive paste has a composition including a plurality of conductive nanoparticles and a plurality of conductive nanowires, wherein a weight ratio of the plurality of conductive nanoparticles to the plurality of conductive nanowires is between about 10:1 and about 50:1. According to further embodiments of the present invention, a method for forming an interconnection and an electrical device are also provided.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.15/547,462, filed 28 Jul. 2017, which is a 371 of InternationalApplication No. PCT/SG2016/050044 filed 29 Jan. 2016, which claims thebenefit of priority of U.S. provisional application No. 62/109,785,filed 30 Jan. 2015, the content of it being hereby incorporated byreference in its entirety for all purposes.

TECHNICAL FIELD

Various embodiments relate to a conductive paste, a method for formingan interconnection and an electrical device.

BACKGROUND

Tin (Sn)-based solders have been the most commonly used lead-free solderfor joining microelectronics devices and components. Even at a lowprocess temperature (200-250° C.), the reactions between the solder andcopper (Cu) bump, Under Bump Metallizations (UBMs), such as copper (Cu),nickel (Ni), silver (Ag), palladium (Pd), etc., are very aggressive andcan cause serious reliability concerns. The formation of intermetalliccompounds (IMCs) results in degradation of mechanical properties due totheir brittle nature, and a decrease in electrical conductivity due totheir higher resistivity than pure metals. The Kirkendall voids whichare formed as a consequence of the imbalanced inter-diffusion betweendifferent metals also deteriorate the mechanical and electricalproperties of the joint. These are the reasons why copper (Cu) has beensuggested as an alternative bonding material instead of Sn-based solder.Indeed, Cu to Cu homogeneous joining provides a solution to avoidconcerns about complex metallic reactions and the accompanying issues,consequently achieving a high reliability bonding. Nevertheless, thereare limitations of Cu to Cu direct bonding. To achieve reliable Cu—Cuthin-film bonding, high temperature and high pressure are required,hence applications to semiconductor processes or electronics packagingis limited.

SUMMARY

According to an embodiment, a conductive paste is provided. Theconductive paste may have a composition including a plurality ofconductive nanoparticles and a plurality of conductive nanowires,wherein a weight ratio of the plurality of conductive nanoparticles tothe plurality of conductive nanowires is between about 10:1 and about50:1.

According to an embodiment, a method for forming an interconnection isprovided. The method may include applying a conductive paste asdescribed herein between a first substrate portion and a secondsubstrate portion, and fusing the plurality of conductive nanoparticlesof the conductive paste to each other to interconnect the firstsubstrate portion and the second substrate portion.

According to an embodiment, an electrical device is provided. Theelectrical device may include a first substrate portion, a secondsubstrate portion, and a conductive member arranged to interconnect thefirst substrate portion and the second substrate portion, wherein theconductive member is made of the conductive paste as described herein,the conductive paste processed to fuse the plurality of conductivenanoparticles of the conductive paste to each other.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to like partsthroughout the different views. The drawings are not necessarily toscale, emphasis instead generally being placed upon illustrating theprinciples of the invention. In the following description, variousembodiments of the invention are described with reference to thefollowing drawings, in which:

FIG. 1A shows a schematic top view of a conductive paste, according tovarious embodiments.

FIG. 1B shows a flow chart illustrating a method for forming aninterconnection, according to various embodiments.

FIGS. 1C and 1D show schematic cross sectional views of an electricaldevice, according to various embodiments.

FIG. 2A shows, as cross-sectional views, various processing stages of amethod of manufacturing copper (Cu) nanowires (NWs) and mixture withcopper (Cu) nanoparticles (NPs), according to various embodiments.

FIG. 2B shows, as cross-sectional views, various processing stages of amethod of manufacturing copper (Cu) nanowires, according to variousembodiments.

FIG. 2C shows scanning electron microscope (SEM) images of coppernanowires (Cu NWs), according to various embodiments.

FIG. 2D shows a transmission electron microscope (TEM) image of coppernanoparticles (Cu NPs) and a schematic diagram of an individual coppernanoparticle, according to various embodiments.

FIG. 2E shows schematic diagrams of a method for washing a copper (Cu)nanoparticle paste, according to various embodiments.

FIG. 2F shows schematic diagrams of a method for forming aninterconnection, according to various embodiments.

FIG. 2G shows photographs illustrating examples of some steps of theprocessing method of various embodiments.

FIG. 3A shows a schematic diagram illustrating chip to substrate bondingusing the copper (Cu) paste of various embodiments, while FIG. 3B showsschematic diagrams of the bonding layer dispersed on the substrate.

FIG. 4A shows schematic diagrams illustrating the strengtheningmechanism of the mixture of nanowires and nanoparticles of variousembodiments.

FIG. 4B shows schematic diagrams illustrating the solvent extractioneffect with nanowires in nanoparticles paste of various embodiments.

FIGS. 5A and 5B show scanning electron microscope (SEM) images of copper(Cu) pillar formed using the nano copper (Cu) paste of variousembodiments.

FIGS. 6A and 6B show scanning electron microscope (SEM) images of copper(Cu) bumps formed from a copper paste with nanowires of variousembodiments, while FIGS. 6C and 6D show scanning electron microscope(SEM) images of copper (Cu) bumps formed from a copper paste withoutnanowires.

FIGS. 7A to 7D show scanning electron microscope (SEM) images of themicrostructures of the fracture surface of the copper (Cu) nanoparticles(NPs) and nanowires (NWs) composite joining bonded at 200° C.

FIG. 7E shows a scanning electron microscope (SEM) image illustratingthe microstructures of the bonding layer formed using the copper pasteof various embodiments.

FIG. 8 shows a plot of shear strengths for various copper (Cu)nanostructures.

FIG. 9A shows a plot of shear strength variation with the length and theweight ratio of the nanoparticles to the nanowires.

FIG. 9B shows a plot of shear strength variation with the length ofnanowires.

FIG. 10A shows a plot of shear strength variation with the length ofnanowires.

FIG. 10B shows a schematic cross sectional view of an electrical device,according to various embodiments.

FIGS. 10C and 10D show scanning electron microscope (SEM) images takenat positions “A” and “B” respectively indicated in FIG. 10B. Each scalebar represents 500 μm.

FIG. 11 shows a plot of in-situ resistance measurements for copperpastes.

DETAILED DESCRIPTION

The following detailed description refers to the accompanying drawingsthat show, by way of illustration, specific details and embodiments inwhich the invention may be practiced. These embodiments are described insufficient detail to enable those skilled in the art to practice theinvention. Other embodiments may be utilized and structural, logical,and electrical changes may be made without departing from the scope ofthe invention. The various embodiments are not necessarily mutuallyexclusive, as some embodiments can be combined with one or more otherembodiments to form new embodiments.

Embodiments described in the context of one of the methods or devicesare analogously valid for the other methods or devices. Similarly,embodiments described in the context of a method are analogously validfor a device, and vice versa.

Features that are described in the context of an embodiment maycorrespondingly be applicable to the same or similar features in theother embodiments. Features that are described in the context of anembodiment may correspondingly be applicable to the other embodiments,even if not explicitly described in these other embodiments.Furthermore, additions and/or combinations and/or alternatives asdescribed for a feature in the context of an embodiment maycorrespondingly be applicable to the same or similar feature in theother embodiments.

In the context of various embodiments, the articles “a”, “an” and “the”as used with regard to a feature or element include a reference to oneor more of the features or elements.

In the context of various embodiments, the phrase “at leastsubstantially” may include “exactly” and a reasonable variance.

In the context of various embodiments, the term “about” as applied to anumeric value encompasses the exact value and a reasonable variance.

As used herein, the term “and/or” includes any and all combinations ofone or more of the associated listed items.

As used herein, the phrase of the form of “at least one of A or B” mayinclude A or B or both A and B. Correspondingly, the phrase of the formof “at least one of A or B or C”, or including further listed items, mayinclude any and all combinations of one or more of the associated listeditems.

FIG. 1A shows a schematic top view of a conductive paste 100, accordingto various embodiments. The conductive paste 100 has a compositionincluding a plurality of conductive nanoparticles 102 and a plurality ofconductive nanowires 104, wherein a weight ratio of the plurality ofconductive nanoparticles 102 to the plurality of conductive nanowires104 is between about 10:1 and about 50:1.

In other words, a conductive paste 100 may be provided. The conductivepaste 100 may include a mixture of a plurality of conductivenanoparticles (NPs) 102 and a plurality of conductive nanowires (NWs)104. The amount of the plurality of conductive nanoparticles (NPs) 102and the plurality of conductive nanowires (NWs) 104 in the conductivepaste 100, or in other words, the composition of the conductive paste100, may be such that the weight ratio of the plurality of conductivenanoparticles 102 to the plurality of conductive nanowires 104 isbetween about 10:1 and about 50:1, for example, between about 10:1 andabout 40:1, between about 10:1 and about 20:1, between about 20:1 andabout 50:1, between about 20:1 and about 40:1, or between about 30:1 andabout 50:1. This may mean that the major/main constituent in theconductive paste 100 is the plurality of conductive nanoparticles 102while the minor constituent in the conductive paste 100 is the pluralityof conductive nanowires 104. The plurality of conductive nanoparticles102 and the plurality of conductive nanowires 104 may be separate ordistinct from one another. This may mean that the plurality ofconductive nanoparticles 102 are generally not fused to each otherwithin the conductive paste 100. The conductive paste 100 may includeresidual solvent(s) (e.g., alcohol-based solvent(s)), for example,resulting from the preparation of the conductive paste 100.

In various embodiments, the plurality of conductive nanowires 104 mayact as a mechanical strengthening material. Further, the plurality ofconductive nanowires 104 may also create pathways in the conductivepaste 100 to allow any residual chemical(s) or solvent(s) that may bepresent in the conductive paste 100 to escape from isolated regions orgaps within the conductive paste 100.

In various embodiments, the weight ratio of the plurality of conductivenanoparticles 102 to the plurality of conductive nanowires 104 may bebetween about 10:1 and about 40:1, for example, between about 10:1 andabout 20:1, between about 20:1 and about 40:1, between about 30:1 andabout 40:1, or between about 10:1 and about 40:1, e.g., about 20:1,about 30:1 or optimally at about 40:1.

In various embodiments, the weight ratio of the plurality of conductivenanoparticles 102 to the plurality of conductive nanowires 104 may beabout 40:1.

While not wishing to be bound by any theory, in various embodiments,such a high weight ratio of the plurality of conductive nanoparticles102 to the plurality of conductive nanowires 104 of between about 10:1and about 50:1, or between about 10:1 and about 40:1, or up to about40:1, or up to about 50:1 (i.e., a much higher amount of nanoparticlesversus nanowires) may be employed because of the selection of the lengthof the conductive nanowires 104. In various embodiments, as will bedescribed further below, the conductive nanowires 104 employed may havea length of between about 20 μm and about 40 μm. Further, whenconductive nanowires 104 of uniform length are used in the conductivepaste 100, a higher shear strength may be observed (see FIGS. 10A-10D tobe described below).

In the context of various embodiments, each conductive nanoparticle 102of the plurality of conductive nanoparticles 102 may have a size (ordiameter) of between about 5 nm and about 20 nm, for example, betweenabout 5 nm and about 15 nm, between about 5 nm and about 10 nm, betweenabout 10 nm and about 20 nm, or between about 8 nm and about 15 nm. Itshould be appreciated that there may be a size distribution or varianceamong the plurality of conductive nanoparticles 102. In someembodiments, the plurality of nanoparticles 102 may have the same (oruniform) size (or diameter).

By having smaller sized nanoparticles (e.g., 5-20 nm, or <10 nm) 102,the processing temperature of a heating process (to be described below)to be carried out for fusion of the plurality of conductivenanoparticles 102 to each other to form an interconnection may be lower(for example 200-280° C.) as compared to larger sized nanoparticles. Forexample, for nanoparticles with 40-100 nm diameter, the processtemperature required may be in the range of 300-350° C. Further, smallersized nanoparticles (e.g., 5-20 nm, or <10 nm) have been found to drivefusion of the conductive nanoparticles 102 to each other, and with theconductive nanowires 104.

In the context of various embodiments, each conductive nanowire 104 ofthe plurality of conductive nanowires 104 may have a length of betweenabout 5 μm and about 50 μm, for example, between about 5 μm and about 40μm, between about 5 μm and about 20 μm, between about 20 μm and about 50μm, or between about 20 μm and about 40 μm, e.g., about 40 μm. Theplurality of nanowires 104 may have the same (or uniform) length.

In the context of various embodiments, each conductive nanowire 104 ofthe plurality of conductive nanowires 104 may have a diameter of betweenabout 100 nm and about 200 nm, for example, between about 100 nm andabout 150 nm, between about 100 nm and about 120 nm, between about 150nm and about 200 nm, or between about 120 nm and about 150 nm. Theplurality of nanowires 104 may have the same (or uniform) diameter.

In the context of various embodiments, each conductive nanowire 104 ofthe plurality of conductive nanowires 104 may have an aspect ratio ofbetween about 50 and about 500, for example, between about 50 and about250, between about 50 and about 100, between about 100 and about 500, orbetween about 100 and about 300. The term “aspect ratio” may mean thelength-to-width ratio or length-to-diameter ratio.

In the context of various embodiments, the plurality of conductivenanoparticles 102 and/or the plurality of conductive nanowires 104 mayinclude a metal. The metal may be selected from the group consisting ofcopper (Cu), silver (Ag) and gold (Au). In various embodiments, theplurality of conductive nanoparticles 102 and the plurality ofconductive nanowires 104 may include or may be made of the same metal.

In the context of various embodiments, the plurality of conductivenanoparticles 102 and the plurality of conductive nanowires 104 mayinclude or consist essentially of copper (Cu). This may mean that copper(Cu) may be utilized as the conductive and joining materialsimultaneously.

In the context of various embodiments, each conductive nanoparticle 102of the plurality of conductive nanoparticles 102 may be encapsulatedwith an organic layer. This may mean that each conductive nanoparticle102 may be coated on its surface with an organic layer. Therefore, theorganic layer may be a capping layer or a surfactant. The organic layermay prevent or minimize oxidation of the material of the conductivenanoparticle 102. The organic layer may prevent or minimizeagglomeration of the conductive nanoparticles 102. The organic layer maybe a polymeric layer. In various embodiments, the organic layer may beremoved or volatized in a heating process, for example, during theheating process for fusing the plurality of conductive nanoparticles 102to each other.

In the context of various embodiments, the organic layer encapsulatingeach conductive nanoparticle 102 may include an amine or an aminecompound, for example, including but not limited to n-heptylamine,n-octylamine, n-nonylamine, or n-decylamine. By capping the conductivenanoparticles 102 with an amine compound, a lower heating temperaturemay be required for effective “necking”/fusing among the conductivenanoparticles 102 and/or with the conductive nanowires 104, as comparedto other capping materials (e.g., polyvinylpyrrolidone (PVP)), whichrequire a higher heating temperature, resulting in a lesser“necking”/fusing among the nanoparticles. Therefore, various embodimentsmay enable better bonding among the conductive nanoparticles 102, and/orwith the conductive nanowires 104, which may lead to better electrical,thermal and mechanical strength in the interconnects.

In various embodiments, the organic amine surfactant on each conductivenanoparticle 102 may include C6-C18 alkylamines. In some embodiments thealkylamine employed may be a C7-C10 alkylamine, although it should beappreciated that a C5 or C6 alkylamine may also be used. Likewise, a C11or C12 alkyl amine may also be used. The exact size of alkylamine to beemployed may be a balance between being long enough to provide aneffective inverse micellar structure versus the ready volatility and/orease of handling. For example, alkylamines with more than 18 carbons,while also usable, may be more difficult to handle because of their waxycharacter. Alkylamines between C7 and C10, inclusive, represent a goodbalance of desired properties for ease of use. In various embodiments,the C6-C18 alkylamine may be at least one of n-heptylamine,n-octylamine, or n-nonylamine. While these are all normal chain amines,one skilled in the art will appreciate that some extent of branching mayalso be used. As a non-limiting example, 7-methyloctylamine may also beused. Without wishing to be bound by theory, the monoalkylaminesdescribed above may also serve as ligands in the coordination sphere ofthe nanoparticles (e.g., copper nanoparticles) 102. However, theirability to dissociate from the copper is facilitated by the single pointof attachment.

Various embodiments may employ an N,N′-dialkylethylenediamine whichincludes a C1-C4 N,N′-dialkylethylenediamine. As bidentate ligands,N,N′-dialkylethylenediamines may coordinate metal atoms at two nitrogenatoms, which may stabilize the formation of small diameter nanoparticles(e.g., Cu nanoparticles). In some embodiments, the alkyl groups of theC1-C4 N,N′-dialkylethylenediamine may be the same, while in otherembodiments, may be different. The C1-C4 alkyl groups may includemethyl, ethyl, propyl, butyl, and the like, including normal chain orbranched alkyl groups such as iso-propyl, iso-butyl, sec-butyl, andtert-butyl groups. Other bidentate, tridentate, and multidentate ligandsmay also be employed. For example, N,N′-dialkylpropylenediamines mayalso be used.

FIG. 1B shows a flow chart illustrating a method 110 for forming aninterconnection, according to various embodiments.

At 112, a conductive paste as described herein (e.g., 100, FIG. 1A) isapplied between a first substrate portion and a second substrateportion. The conductive paste may be applied to one or both of the firstsubstrate portion and the second substrate portion.

At 114, the plurality of conductive nanoparticles of the conductivepaste are fused to each other to interconnect the first substrateportion and the second substrate portion. In various embodiments, theplurality of conductive nanoparticles of the conductive paste may alsobe fused to the plurality of conductive nanowires of the conductivepaste.

In the context of various embodiments, the term “interconnection” maymean an electrical interconnection and/or a bonding layer.

By providing the conductive paste between the first substrate portionand the second substrate portion, and subsequently fusing the pluralityof conductive nanoparticles of the conductive paste to each other, thefirst substrate portion and the second substrate portion may beelectrically interconnected to each other.

In various embodiments, fusing the plurality of conductive nanoparticlesof the conductive paste to each other may interconnect and bond thefirst substrate portion and the second substrate portion to each other.

In various embodiments, at 114, fusing the plurality of conductivenanoparticles of the conductive paste to each other may form aconductive member to interconnect the first substrate portion and thesecond substrate portion to each other. This may mean that a conductivemember may be formed from the conductive paste after fusing theplurality of conductive nanoparticles in the conductive paste to eachother. The conductive member may be a composite material having theplurality of conductive nanoparticles fused to each other and theplurality of conductive nanowires (that may be fused to the plurality ofconductive nanoparticles). The conductive member may be in the form of abump or a pillar.

As described above, the major/main constituent of the conductive pasteis the plurality of conductive nanoparticles while the minor constituentof the conductive paste is the plurality of conductive nanowires.Therefore, the plurality of conductive nanoparticles are the majormaterial for bonding and/or interconnection, and the plurality ofconductive nanowires are the assistant material for improving propertiesof the bonding and/or interconnection.

The plurality of conductive nanowires may act as a mechanicalstrengthening material. For example, the plurality of conductivenanowires may act as barriers to the propagation of crack(s) that mayoccur in the fused plurality of conductive nanoparticles or in theconductive member, and therefore may retard the increase in the crackarea, thereby improving the mechanical properties. The plurality ofconductive nanowires may also create pathways in the conductive pasteand/or the conductive member to allow any residual chemical(s) orsolvent that may be present to escape from isolated regions or gapswithin the conductive paste and/or the conductive member.

In various embodiments, at 114, in order to fuse the plurality ofconductive nanoparticles of the conductive paste to each other, theconductive paste may be subjected to a heating process. This may meanthat a heat treatment may be carried out to fuse the plurality ofconductive nanoparticles to each other. This also means that the heatingprocess is carried out after the conductive paste has been appliedbetween the first substrate portion and the second substrate portion. Asthe conductive paste is subjected to the heating process, the pluralityof conductive nanoparticles and plurality of conductive nanowires aresubjected to the heating process. The heating process may be carried outin a tube furnace, a vacuum oven, a reflow oven (optimum), etc. Theenvironment in which the heating process may be performed may include aninert gas (nitrogen, N₂, or argon, Ar) (optimum) or may be a vacuum.

In various embodiments, the heating process may be carried out for apredetermined duration between about 6 minutes and about 30 minutes, forexample, between about 6 minutes and about 20 minutes, between about 6minutes and about 10 minutes, between about 10 minutes and about 30minutes, between about 10 minutes and about 20 minutes, or between about6 minutes and about 8 minutes, e.g., optimally for about 6 minutes.

In various embodiments, a predetermined peak (or maximum) temperature ofthe heating process may be between about 200° C. and about 350° C., forexample, between about 200° C. and about 300° C., between about 200° C.and about 280° C., between about 200° C. and about 250° C., betweenabout 250° C. and about 350° C., between about 250° C. and about 280°C., or between about 220° C. and about 250° C., e.g., about 200° C. oroptimally at about 280° C. Accordingly, therefore, a low temperatureprocessing method may be provided.

In various embodiments, the heating process at the predetermined peaktemperature may be carried out for a predetermined duration of betweenabout 90 seconds and about 10 minutes, for example, between about 90seconds and about 5 minutes, between about 90 seconds and about 3minutes, between about 3 minutes and about 10 minutes, between about 3minutes and about 5 minutes, between about 5 minutes and about 10minutes, or between about 2 minutes and about 4 minutes, e.g., optimallyfor about 90 seconds. As non-limiting examples, in a tube furnace, forheating at a predetermined peak temperature of about 200-350° C., thepredetermined duration may be about 10 minutes, while in a reflow oven,for heating at a predetermined peak temperature of about 200-300° C.,the predetermined duration may be about 90 seconds.

In various embodiments, the heating process may volatize the residualsolvent(s) that may be present or trapped in the conductive paste and/orthe capping layer(s) (e.g., chemicals or organic layer(s)) that may bepresent on the plurality of conductive nanoparticles so as to achieve abetter bonding performance.

In various embodiments, no mechanical pressure is applied tointerconnect and/or bond the first substrate portion and the secondsubstrate portion to each other. This may mean that no mechanicalpressure is necessarily required to be applied in order to interconnectand/or bond the first substrate portion and the second substrate portionto each other. However, it should be appreciated that, in someembodiments, mechanical pressure may be applied.

In various embodiments, the first substrate portion and the secondsubstrate portion may be comprised in a single continuous substrate.

In various embodiments, the first substrate portion and the secondsubstrate portion may be respectively comprised in separate substratesarranged one over the other.

In various embodiments, the method may include forming the conductivepaste including providing a plurality of conductive nanoparticles,providing a plurality of conductive nanowires, and mixing the pluralityof conductive nanoparticles and the plurality of conductive nanowires ina solvent. The solvent may include an alcohol. The alcohol may includeat least one of isopropyl alcohol (IPA), pentane, heptanol, or hexanol.Heptanol or hexanol may be used for optimum conditions.

In various embodiments, at least some, or most, of the solvent may beremoved, evaporated or volatized during the heating process for fusingthe plurality of conductive nanoparticles to each other. The heatingtemperature of the heating process is higher than the boiling point ofthe solvent.

In various embodiments, the plurality of conductive nanowires may beprovided by forming the plurality of conductive nanowires by means of anelectroplating method using an anodic aluminum oxide (AAO) as atemplate. The anodic aluminum oxide (AAO) may include holes or pores orchannels into which the material for the plurality of conductivenanowires may be electroplated to form the plurality of conductivenanowires. The plurality of conductive nanowires may then be extractedor removed from the anodic aluminum oxide template. By employing ananodic aluminum oxide (AAO) (or anodized aluminum oxide (AAO)) as atemplate, a uniform distribution of the size (e.g., length and/ordiameter) of the plurality of conductive nanowires may be obtained.

It should be appreciated that, in general, the method may includemixing, where the plurality of conductive nanoparticles and theplurality of conductive nanowires may be mixed (e.g., in a solvent), andthereafter, dispersing the mixed paste onto at least one substrate (orsubstrate portion) and followed by heating of the conductive paste, forexample, to fuse the plurality of conductive nanoparticles to eachother. The plurality of conductive nanoparticles may also fuse to theplurality of conductive nanowires.

While the method described above is illustrated and described as aseries of steps or events, it will be appreciated that any ordering ofsuch steps or events are not to be interpreted in a limiting sense. Forexample, some steps may occur in different orders and/or concurrentlywith other steps or events apart from those illustrated and/or describedherein. In addition, not all illustrated steps may be required toimplement one or more aspects or embodiments described herein. Also, oneor more of the steps depicted herein may be carried out in one or moreseparate acts and/or phases.

FIGS. 1C and 1D show schematic cross sectional views of an electricaldevice 120 a, 120 b, according to various embodiments. The electricaldevice 120 a, 120 b includes a first substrate portion 122 a, 122 b, asecond substrate portion 124 a, 124 b, and a conductive member 126 a,126 b arranged to interconnect the first substrate portion 122 a, 122 band the second substrate portion 124 a, 124 b, wherein the conductivemember 126 a, 126 b is made of the conductive paste as described herein(e.g., 100, FIG. 1A), the conductive paste processed (e.g., heattreated) to fuse the plurality of conductive nanoparticles (e.g., 102,FIG. 1A) of the conductive paste to each other.

In other words, an electrical device 120 a, 120 b may be provided. Theelectrical device 120 a, 120 b may include a first substrate portion 122a, 122 b, a second substrate portion 124 a, 124 b, and a conductivemember arranged in between the first substrate portion 122 a, 122 b andthe second substrate portion 124 a, 124 b. The plurality of conductivenanoparticles of the conductive paste may be fused to each other(resulting in the conductive member 126 a, 126 b) to interconnect thefirst substrate portion 122 a, 122 b and the second substrate portion124 a, 124 b to each other. Therefore, the conductive member 126 a, 126b may electrically interconnect the first substrate portion 122 a, 122 band the second substrate portion 124 a, 124 b to each other. In variousembodiments, the conductive member 126 a, 126 b may electricallyinterconnect and bond the first substrate portion 122 a, 122 b and thesecond substrate portion to each other 124 a, 124 b. In this way, theconductive member 126 a, 126 b may act as an electrical interconnectionand/or a bonding member.

In various embodiments, within the conductive member 126 a, 126 b, theplurality of conductive nanoparticles of the conductive paste may alsobe fused to the plurality of conductive nanowires of the conductivepaste.

In various embodiments, the conductive member 126 a, 126 b may be acomposite material having the plurality of conductive nanoparticlesfused to each other and the plurality of conductive nanowires (that maybe fused to the plurality of conductive nanoparticles).

The plurality of conductive nanowires in the conductive member 126 a,126 b may act as a mechanical strengthening material. For example, theplurality of conductive nanowires may act as barriers to the propagationof crack(s) that may occur in the fused plurality of conductivenanoparticles in the conductive member 126 a, 126 b and therefore mayretard the increase in the crack area, thereby improving the mechanicalproperties.

In various embodiments, each of the first substrate portion 122 a, 122 band the second substrate portion 124 a, 124 b may include a conductiveportion or an electrical circuit. The conductive member 126 a, 126 b maybe arranged to interconnect the respective conductive portions(electrical circuits) of the first substrate portion 122 a, 122 b andthe second substrate portion 124 a, 124 b to each other.

As shown in FIG. 1C, the first substrate portion 122 a and the secondsubstrate portion 124 a may be comprised in a single continuoussubstrate 128 a. This may mean that the first substrate portion 122 aand the second substrate portion 124 a may be part of a singlecontinuous substrate 128 a, and therefore refer to separate portions ofthe single continuous substrate 128 a.

As shown in FIG. 1D, the first substrate portion 122 b and the secondsubstrate portion 124 b may be comprised in separate substrates arrangedone over the other. This may mean that the first substrate portion 122 band the second substrate portion 124 b may be separate substratesthemselves or may be part of respective separate substrates.

In various embodiments, the conductive member 126 a, 126 b may be or mayinclude a bump or a pillar.

In the context of various embodiments, the term “conductive” may include“electrically conductive” and/or “thermally conductive”.

In the context of various embodiments, the terms “fuse” and “fusing” maymean sintering, or joining together as an (single) entity. This may meanthat there may not be clear or obvious boundary observable between twomaterials (or structures) when the two materials are fused to eachother. Further, the two materials fused to each other may not beseparate or distinct.

It should be appreciated that descriptions in the context of theconductive paste 100 and the electrical device 120 a, 120 b may becorrespondingly applicable to each other, and may also becorrespondingly applicable in relation to the method for forming aninterconnection, and vice versa.

Various embodiments may provide a copper nanoparticles-nanowires mixturefor electronics joining. The copper nanoparticle-nanowire mixture mayprovide a solution or approach for low temperature and low pressurebonding in microelectronics applications.

Various embodiments may include the addition of copper nanowires (CuNWs) to copper nanoparticles (Cu NPs) as a mechanical strengtheningmaterial. At low temperature bonding at about 200° C., the nanoscale NPsdo not show sufficient mechanical reliability. Although they havestarted to neck and fuse, the sintered Cu nanoparticles may still havepores between them and may not achieve 100% densification. Thus, cracksmay propagate easily along the fused regions of the NPs. As growth of acrack area leads to a reduction in bonding area, the mechanicalproperties, as well as the electrical and thermal properties, may beseverely degraded with crack propagation. In various embodiments, theadded Cu nanowires behave as a barrier to crack propagation in the fusedCu nanoparticles and retard the increase in crack area.

Furthermore, the Cu nanowires have another important role besidesstrengthening the nanoparticles joint. The solvent that is mixed withthe Cu nanoparticles paste should be completely removed throughvolatilization above a certain temperature for effective sintering ofthe nanoparticles. However, much of the solvent may remain trappedaround the Cu nanoparticles and this may degrade the mechanical andelectrical properties of the joint. This will be especially true forlarge joints where the solvents have a much larger distance to travelbefore reaching a “free” surface. In various embodiments, the additionalCu nanowires introduced create pathways through which residual chemicals(e.g., solvents) may escape from the isolated regions. Therefore, thetwo effects described above may enable Cu NPs bonding to be much morestable and reliable, which may allow it to be extended to various formsof joining in electronics applications.

As described above, copper-to-copper (Cu-to-Cu) homogeneousinterconnection with copper nanoparticles (Cu NPs) or paste is anattractive approach to make an electrical joint at low temperature andwithout the use of pressure. In various embodiments, Cu is utilized asthe conductive and joining material simultaneously. By using only Cu asa metallization, bump and joining material, aggressive inter-diffusionand reaction between heterogeneous materials can be avoided. For thatreason, some researchers proposed Cu direct bonding with Cu paste, butthe process still requires high temperature to achieve the acceptablemechanical strength. There were attempts to add wires, fibers orwhiskers with the particles and power sintering to achieve lowtemperature high stability bonding. A recent research has been reportedon electrode and interconnecting materials using silver (Ag) nanowires,where the manufacturing process of the nanowires is by chemicalsynthesis, and thus the nanowires are not refinable and controllable.However, the present inventors have found that the length anddistribution of nanowires are very sensitive and may be criticalvariables in the properties of the NP—NW mixture, and in the mechanicaland electrical properties of bonding or interconnection. In other words,in order to accomplish highly stable and reliable bonding with metallicnanowires (NWs), it may be required to utilize uniform length, diameterand sufficiently long nanowires. In various embodiments, the coppernanowires (Cu NWs) may be grown to the desired length and added tonanoparticles (NPs) in an optimal ratio. As a result, a high reliablejoining in mechanical property, even at a low temperature (200° C.), maybe achieved.

In various embodiments, copper nanoparticles (Cu NPs) and coppernanowires (Cu NWs) are mixed at an appropriate ratio (e.g., weightratio) to address the above-mentioned challenges. In variousembodiments, Cu NPs are employed as the main conductive and joiningmaterial with the Cu NWs as a strengthening material between the NPs.

In various embodiments, the copper (Cu) nanoparticles employed invarious embodiments may be chemically synthesized with size <10 nm, thathave a protective polymeric layer. The copper nanoparticles (Cu NPs) maybe formed using the method described in PCT/US2010/039069, the entiredisclosure of which is incorporated herein by reference.

FIG. 2A shows, as cross-sectional views, various processing stages of amethod 240 of manufacturing copper (Cu) nanowires (NWs) and mixture withcopper (Cu) nanoparticles (NPs), according to various embodiments,illustrating a procedure of manufacturing of Cu NWs using anodizedaluminum oxide (AAO). In general, gold/copper (Au/Cu) layers may bedeposited on the AAO as a seed layer for electroplating. Cu maysubsequently be electroplated into the holes in the AAO. Then, the seedlayers and the AAO may be etched away by the respective chemicaletchants. Finally, the Cu NWs may be detached. As the Cu nanowires areproduced through electroplating using anodic aluminum oxide (AAO) as atemplate, a narrow distribution in the length of the NWs may be obtainedand the length of NWs and the weight ratio of NPs/NWs may be controlled.While it is described herein that the electroplating method with AAO asa template may be used to obtain the Cu nanowires, it should beappreciated that uniform Cu nanowires which may be produced by othermethods such as chemical synthesis, vapor-liquid-solid method orchemical vapor deposition may also be applied in various embodiments.However, NWs which have a broader variation in length and radius may beless effective and the process window could be wider in optimizing theratio and maximum effect. Finally, the Cu NWs may be added to Cu NPs andthe two materials may be mixed in a solvent, and then washed with analcohol based solution.

As a non-limiting example, referring to FIG. 2A, for producingnanowires, an anodic aluminum oxide (AAO) template 242 may first beprovided or prepared. The AAO template 242 may have a plurality of holes(or pores or channels) 243. The AAO template 242 may be sputtered withAu and Cu, where a 0.2 nm thick gold layer 244 and a 1 μm thick copperlayer 246 may be obtained.

The AAO template 242 may then be attached onto a cathode (not shown) and50 μm long Cu nanowires 204 may be electrochemical synthesized in theholes 243 of the AAO template 242.

The AAO template 242 may be attached to a protective film (e.g., athermal tape) 248. The sputtered Cu layer 246 and Au layer 244 may beetched away by chemical etching processes. The AAO template 242 may beetched using a sodium hydroxide (NaOH) solution. As a result, free Cunanowires 204 may be obtained. The Cu nanowires 204 may then be washedwith ethanol, followed by isopropyl alcohol (IPA).

FIG. 2B shows, as cross-sectional views, various processing stages of amethod 240 b of manufacturing copper (Cu) nanowires, according tovarious embodiments. An anodic aluminum oxide (AAO) template 242 b,having a plurality of holes (or pores or channels) 243 b, may beprovided or prepared. The AAO template 242 b may be sputtered with a 500nm thick copper (Cu) layer 246 b (as a seed layer). The AAO template 242b may then be attached onto a cathode (not shown) and 50 μm long Cunanowires 204 b may be electrochemical synthesized in the holes 243 b ofthe AAO template 242 b. The AAO template 242 b may then be removed, forexample, by etching using a sodium hydroxide (NaOH) solution. As aresult, Cu nanowires 204 b may remain on the Cu layer 246 b.Subsequently, the Cu nanowires 204 may be removed from the Cu layer 246b and washed.

FIG. 2B also shows scanning electron microscope (SEM) images of coppernanowires (Cu NWs) 204 c that are formed using an AAO template 242 c.Examples of copper nanowires (Cu NWs) that are formed, after removal ofthe AAO template, are shown in the scanning electron microscope (SEM)images of FIG. 2C illustrating Cu NWs having lengths of about 15 μm,about 25 μm and about 45 μm.

As described, fine Cu nanowires may be formed by electroplating usinganodic aluminum oxide (AAO). The diameter and length of the nanowiresmay be adjusted depending on the AAO (hole) size and/or the platingcondition(s). For example, the length of the nanowires may be adjustedeasily up to about 50 Nanowires with extremely high aspect ratio (e.g.,about 50-250) may be achieved. Further, nanowires such as Cu nanowiresgrown using AAO may exhibit very uniform diameter and length. Growingnanowires using AAO offers a fast, simple and low cost process.

Referring back to FIG. 2A, Cu nanowires 204 and Cu nanoparticles 202 maybe mixed together for forming a conductive paste. As non-limitingexamples, for preparing a mixture of nanoparticles and nanowires, about2 g of copper nanoparticles may be weighed and may be provided into asample holder. A washing solution (e.g., alcohol based solvent) may beadded into the sample holder. The sample containing the coppernanoparticles and the solvent may be placed in an ultrasonic bath forabout 30 seconds. About 0.04 g of copper nanowires may then be weighedand added into the sample holder, with the sample subsequently placed inan ultrasonic bath for about 30 seconds. The sample or mixture may becentrifuged, and the solution or solvent drained from the sample holder.Optionally, another round of washing may be performed, by adding anotherwashing solution (e.g., alcohol based solvent) into the sample holder,where the solvent may be subsequently removed. The resulting paste ofcopper nanoparticles and copper nanowires may be transferred to asyringe to facilitate deposition of the paste.

FIG. 2D shows a transmission electron microscope (TEM) image of coppernanoparticles (Cu NPs) 202 d that may be used in various embodiments.The copper nanoparticles (Cu NPs) 202 d may be formed, for example,following the method described in PCT/US2010/039069. FIG. 2D furthershows a schematic diagram of an individual copper nanoparticle 202 d,which may be encapsulated with an organic layer (e.g., a polymericlayer) 203. The organic layer 203 may include an amine or an aminecompound.

FIG. 2E shows schematic diagrams of a method for washing a (nano) copper(Cu) nanoparticle paste, according to various embodiments illustratingthe washing process of the nanoparticles. A copper nanoparticle paste200 e having copper nanoparticles (no nanowires in the nanoparticlepaste 200 e) may be dispensed from a syringe 261 e into a tube 262 e. Asolvent (e.g., alcohol based solvent) 263 e may be dispensed from adispenser 264 e into the tube 262 e. The solution or mixture 265 econtaining the copper nanoparticle paste 200 e and the solvent 263 e maythen be stirred and mixed using an ultrasonicator for about 30 seconds.The mixture 265 e may then be centrifuged and decanted, where, as aresult, the Cu nanoparticle paste 200 e may be at the bottom of the tube262 e to allow the liquid in the mixture 265 e to be removed. One ormore additives 266 e may be dispensed from a dispenser 267 e to be addedinto the tube 262 e to adjust the viscosity of the Cu nanoparticle paste200 e. Subsequently, the Cu nanoparticle paste 200 e may be mixed usingtwo syringes 268 e, 269 e, for example, by moving the Cu nanoparticlepaste 200 e between the syringes 268 e, 269 e. Thereafter, coppernanowires may be mixed in with the washed Cu nanoparticle paste 200 e,for example, as may be described below with reference to FIG. 2F. Ingeneral, the overall process flow may be nanoparticles (rawmaterial)→washing→mixing with nanowires→dispersing (e.g., on asubstrate)→heating.

The additive(s) (for example called a mixing matrix) 266 e may includeone or more organic solvents such as a hydrocarbon, an alcohol, anorganic acid or an amine, or a mixture thereof. As non-limitingexamples, the alcohol may include butanol, octanol, propanol, nonanol,decanol. The additive(s) 266 e, additionally or alternatively, mayinclude glycerols like propylene glycol, glycerol, and/or a surfactantsuch as span20 (sorbitan monolaurate).

FIG. 2F shows schematic diagrams of a method for forming aninterconnection, according to various embodiments. Copper (Cu)nanoparticles 202 f may be dispersed in an alcohol based solvent 257 fin a container 258 f Copper (Cu) nanowires 204 f may be added into thealcohol based solvent 257 f to form a solution 259 f containing the Cunanowires 204 f, the Cu nanoparticles 202 f and the alcohol basedsolvent 257 f, which may then be mixed by an ultrasonicator. Then, thesolution 259 f may be centrifuged and the alcohol based solvent 257 fmay be removed or drained off, for example, using a syringe 260 f, so asto leave behind a conductive paste 200 f in the container 258 f Theconductive paste 200 f, or part thereof, may then be deposited ordispersed between a substrate 224 f and a chip 222 f, for example,dispersed onto the substrate 224 f. A heating process may then becarried out (e.g., at a temperature between about 200° C. and about 280°C.), for example, from below the substrate 224 f. As a result of theheating process, the Cu nanoparticles 202 f in the conductive paste 200f may fuse to one another, resulting in the formation of a conductivemember between the substrate 224 f and the chip 222 f, where theconductive member may electrically interconnect and bond the substrate224 f and the chip 222 f to each other. Also, as a result of the heatingprocess, any residual alcohol based solvent 257 f that may be present inthe conductive paste 200 f may be evaporated.

In various embodiments, the weight ratio of the Cu nanoparticles 202 fto the Cu nanowires 204 f may be between about 10:1 and about 40:1,where the weight ratio of about 40:1 is an optimum weight ratio. Theheating process may be carried out to a peak temperature of betweenabout 200° C. and about 280° C., where the heating duration at the peaktemperature may be about 90 seconds.

FIG. 2G shows photographs illustrating examples of some steps of theprocessing method of various embodiments. FIG. 2G shows a copper paste(NPs+NWs) 200 g provided in a syringe 270 g, from which the copper paste200 g may be applied onto a substrate 224 g. The copper paste 200 g maybe dispersed on the substrate 224 g. An additional substrate (not shown)may be placed over the substrate 224 g with the copper paste 200 g inbetween and subjected to a heating process in a heating apparatus 271 gto interconnect the substrate 224 g with the additional substrate.

It should be appreciated that the methods or steps described in thecontext of FIGS. 2A-2G respectively may be applicable also to othermethods or steps of FIGS. 2A-2G, or may be combined in any manner.

In various embodiments, the copper paste (e.g., 200 f, FIG. 2F) ofvarious embodiments may be stable for more than 6 months. Further, lowtemperature (<200° C.) and pressureless bonding between substrates maybe performed using the copper paste of various embodiments.

In various embodiments, the conditions for interconnection or bondingmay include:

-   -   solvent for mixing nanoparticles (NPs) and nanowires (NWs) may        be an alcohol based solvent such as isopropyl alcohol (IPA),        pentane, heptanol (optimum condition), hexanol (optimum        condition), etc.;    -   weight ratio of NPs:NWs=20:1, 30:1, 40:1 (optimum condition);    -   peak heating temperature of about 200 to about 280° C. (optimum        condition);    -   heating duration of about 6 minutes (optimum condition) to about        30 minutes;    -   heating duration of about 90 seconds (optimum condition) to        about 10 minutes at the peak heating temperature;    -   heating environment may include an inert gas (N₂ or Ar) (optimum        condition) or vacuum;    -   heater apparatus may include tube furnace, reflow oven (optimum        condition), vacuum oven, etc.    -   free of mechanical pressure for bonding (optimum condition).

FIG. 3A shows a schematic diagram illustrating chip 322 to substrate 324bonding using the copper (Cu) paste (NPs+NWs) of various embodiments. Inthis way, the chip or package 322 may be assembled on the substrate 324using the paste (or composite material) containing nanoparticles 302 andnanowires (not shown in FIG. 3A) to form a bonding layer 326. Thebonding layer 326 may be a conductive member (foormed from theconductive paste) that interconnects the chip 322 and the substrate 334.

FIG. 3B shows schematic diagrams of the copper (Cu) composite materialsof Cu NPs+NWs, as a composite bonding layer (or conductive member) 326,dispersed on the substrate 334 (with the chip 322 removed for easierunderstanding and clarity purposes). FIG. 3B also shows an enlarged viewof a part of the copper (Cu) bonding layer 326 to illustrate themicrostructure of the mixed Cu NWs 304 and Cu NPs 302. While not clearlyillustrated, the NPs 302 are fused to each other. The NPs 302 may alsobe fused to the NWs 304.

FIG. 4A shows schematic diagrams illustrating the strengtheningmechanism of the mixture of nanowires 404 and nanoparticles 402 ofvarious embodiments, illustrating the mechanism in which the Cu NWs 404may prevent crack propagation and reinforce the mechanical strength ofthe bond.

In various embodiments, in spite of the nanoscale Cu particle size, low(˜0.2 T_(m)) process temperature and short annealing time (<1 hr) maynot lead to 100% densification. As a result, the area of grain boundaryand the density of voids may not be eliminated completed. Thus, therewould likely be crack propagation path(s) which may lead to thedegradation of mechanical strength. Nevertheless, as shown in FIG. 4A,the Cu NWs 404 which are inserted into the Cu NPs 402 may obstruct thepropagation of crack and may retard crack growth, thereby improving themechanical properties. Therefore, the presence of Cu NWs 404 in theconductive member of various embodiments, after heating of theconductive Cu paste, may improve the mechanical properties of theconductive member by minimising the propagation of crack(s) that mayoccur in the conductive member.

FIG. 4B shows schematic diagrams illustrating the solvent extractioneffect with nanowires 404 b in the Cu nanoparticles bonding layer 426 bof various embodiments to illustrate the second effect of NWs in Cu NPsbonding. In Cu NPs bonding, the paste may contain different kinds ofsolvents as well as organic capping layer around the Cu NPs. However,after sintering by heating, some of the solvents 475 a may still remainand may be trapped in the sintered Cu NPs layer, between the Cu NPs 402b, as shown in FIG. 4B. On the other hand, in the NP+NW compositebonding layer 426 b, the nanowires 404 b may make slits in the isolatedregion and act as diffusion pathways for the chemicals or solvents 475 bto be removed into the surrounding, as shown in FIG. 4B.

The conductive copper paste of various embodiments may be used forcopper (Cu) pillar bump bonding for 3D integration (see, for example,FIGS. 5A and 5B). The conventional method for Cu pillar formation is touse electroplating, which typically takes a long time (0.5-1 hr). Thus,substitute processes have been investigated, and screen printing or inkjet printing method using Cu paste is considered as one of the strongcandidates. Nevertheless, one of the biggest concerns of Cu bumpformation using Cu paste is cracking in the bumps during sintering. Manyof the previous studies reported that as the thickness of the film(bump) or coating increases, more cracks are generated and propagated.Therefore, the formation of thick Cu pillar (about 10-100 μm) withoutcracks is the challenge for Cu pillar bump bonding using nanoparticles.

Two types of Cu bump formations were made using Cu paste: nano Cuparticles and nano Cu particles with nanowires. As shown in FIGS. 5A and5B, Cu pillars (or Cu conductive members) 526 formed using the nanowiresimbedded nano Cu paste of various embodiments has reduced crackgeneration and propagation significantly.

FIGS. 6A and 6B show scanning electron microscope (SEM) images of copper(Cu) bumps 626 formed from a copper paste with nanoparticles 602 andnanowires 604 of various embodiments, while FIGS. 6C and 6D showscanning electron microscope (SEM) images of copper (Cu) bumps 627formed from a copper paste without nanowires. As shown in FIGS. 6A and6B, scanning electron microscope (SEM) analysis reveals the failuresurface of the chip and shows that the nanowires (NWs) 604 mixed wellwith the fused NPs 602. It also shows that the NWs 604 bridge across thecracked regions within the bonded NPs 602. It may be suggested thatthese NWs connections improve the mechanical properties as well as theelectrical properties. In contrast, as shown in FIGS. 6C and 6D wherenanowires are absent, cracks 628 may propagate through the Cu bumps 627.

FIGS. 7A to 7D show scanning electron microscope (SEM) images of themicrostructures of the fracture surface of the copper (Cu) nanoparticles(NPs) and nanowires (NWs) composite joining bonded at 200° C. As shownin FIGS. 7A-7D, Cu NWs 704 (the ellipses shown in FIG. 7A indicate wheresome NWs are) are mixed with the fused NPs 702.

FIG. 7E shows a scanning electron microscope (SEM) image illustratingthe microstructures of the bonding layer formed using the copper pasteof various embodiments. A copper nanowire 704 may be clearly observedamong the fused copper nanoparticles 702.

Mechanical chip shear tests using a shear test machine was carried outand the shear strengths of three different samples: NP only, NP+NW,NP+flake (˜100 μm) samples were compared. FIG. 8 shows a plot 880 ofshear strengths for various copper (Cu) nanostructures, illustratingshear strength variation with nano-structures. Plot 880 shows result 882for the sample with nanoparticles only, result 884 for the sample withnanoparticles and nanowires, and result 886 for the sample withnanoparticles and flakes. The SEM images of the fracture surface of thecorresponding nanostructures are also shown in FIG. 8. The results ofthe shear tests show that the mixture which was added with NWs show thehigher value (result 884) in bonding strength. As shown in FIG. 8, theshear strength of the NP+NW composite sample (result 884) is twice ashigh as for the other samples. The NP+flake mixture was also applied tocompare the effect of large particle sizes (˜100 μm) present in thecomposite, but unlike the NP+NW sample, it does not work as the presenceof the flakes does not increase the shear strength as compared to thesample having nanoparticles only. That is, the composite strengtheningeffect can only be seen in small size long structures such as thenanowires.

In various embodiments, Cu nanowires with a diameter of about 100-200 nmand a length of about 20-50 μm (aspect ratio: about 100-500) and Cunanoparticles with a diameter of about 5-20 nm may be used. The weightratio of Cu NPs to Cu NWs is from about 1:1 to about 50:1, for example,between about 10:1 and about 50:1. Examining a range of size, length,ratio of Cu NWs to nanoparticles, an optimal composition may bedetermined for the required mechanical properties and electricalproperties. Accordingly, the process temperatures may be variedsensitively for better bonding performance depending on the nanoparticlesize. It has been found that with larger Cu nanoparticles between about40 to 100 nm, the same ‘nanowire effect’ may be achieved, but theoptimized process temperature increase to about 300-350° C. Therefore,even though different sizes of nanoparticles may be employed in variousembodiments, only the bonding conditions would be modified, but theconcept of materials and property improvement mechanism are preserved.

The relationship between the configuration of nanowires which containsome nanoparticles and their mechanical property may be investigated aswell. While there was a study on the relationship between the fractionof silver (Ag) nanowires to particles and the mechanical strength, thelength of the chemical synthesized nanowires are not even or uniform.For this reason, it is difficult to determine if the results of thestudy demonstrate the relationship between the fraction of nanowires andthe mechanical strength. Nevertheless, nanowires which are synthesizedby chemical or vapor methods may still be applicable in variousembodiments, because even though they have less effect, it may still beapplicable in terms of the methodology and mechanism.

FIG. 9A shows a plot 980 of shear strength variation with the length andthe weight ratio of the nanoparticles to the nanowires. Plot 980 showsresult 981 for a sample without nanowires, results 982 and 983 forsamples with nanowires having lengths of about 20 μm and about 40 μmrespectively for a weight ratio of nanoparticles to nanowires of about20:1, and results 984 and 985 for samples with nanowires having lengthsof about 20 μm and about 40 μm respectively for a weight ratio ofnanoparticles to nanowires of about 40:1. As shown in FIG. 9A, theweight ratio of nanowires to nanoparticles may affect the mechanicalbonding strength. Through the mechanical tests with different fractions(or weight ratios) of NWs to NPs, it is found that, at a certain ratioof NPs:NWs, the enhancement effect may be maximized. Besides, under andover the certain ratio, the effect of NW addition becomes smaller. Invarious embodiments, it is found that the effective weight ratio ofNPs:NWs is 40:1 for optimum mechanical bonding strength.

When the ratio of NPs:NWs is fixed at the optimized condition (i.e.,40:1), the optimum length of the nanowires may be determined. FIG. 9Bshows a plot 990 of shear strength variation with the length ofnanowires, with the weight ratio of the nanoparticles to the nanowiresfixed at 40:1. Plot 990 shows result 991 for a sample without nanowires,result 992 for a sample with nanowires having a length of about 5 μm,result 993 for a sample with nanowires having a length of about 15 μm,result 994 for a sample with nanowires having a length of about 20 μm,and result 995 for a sample with nanowires having a length of about 40μm. As shown in FIG. 9B, the bonding layer containing longer nanowiresshows a higher bonding strength. The effect of the length of thenanowires could be verified more accurately with electroplated nanowiresas the lengths of the electroplated nanowires are at least substantiallyuniform.

FIG. 10A shows a plot 1080 of shear strength variation with the lengthof nanowires. Three different sample pastes were examined: withoutnanowire (result 1081), mixed with two different lengths (10 μm and 40μm) of nanowires (result 1082) and mixed with uniform length (40 μm) ofnanowires (result 1083), with the ratio of the nanoparticles to thenanowires fixed at about 40:1 by weight. The samples were bonded viaheating at about 230° C. (peak temperature) for about 90 seconds (peaktime duration). The effect of the distribution of the length of thenanowires may be determined, which as shown in FIG. 10A, the sample withnanowires (results 1082, 1083) show a higher shear strength than thesample with no nanowire (result 1081), and the sample having uniformlength nanowires (result 1083) shows a higher shear strength than thatof the sample having mixed lengths (result 1082).

FIG. 10B shows a schematic cross sectional view of an electrical device1020, according to various embodiments, illustrating a bonding structurewith a nano copper (Cu) paste 1026. The nano copper (Cu) paste 1026 maybe provided between a first substrate portion (e.g., silicon (Si) chip)1022 having a copper layer 1023 and a second substrate portion (e.g.,silicon (Si) substrate) 1024 having a copper layer 1025, where the paste1026 may be heat treated to interconnect and/or bond the first substrateportion 1022 and the second substrate portion 1024 to each other.

FIGS. 10C and 10D show scanning electron microscope (SEM) images takenat positions “A” and “B” respectively indicated in FIG. 10B. FIG. 10Cshows SEM images of the surface morphology, at position “A”, after sheartest for three different samples corresponding to those employed forobtaining the results of FIG. 10A. The sample without nanowires shows aflat and clean interface failure mode, and the samples with mixed lengthnanowires and uniform length nanowires show mixed and cohesive failuremodes respectively. These results may be explained that the nanowiresprevent crack propagation, and long and uniform nanowires are moreeffective than mixtures of nanowires with different lengths. Theseresults correspond well with the shear strength results of FIG. 10A.

FIG. 10D shows SEM images of the surface morphology of the surface, atposition “B”, after dispersion of the nano Cu paste 1026. The samplewithout nanowires shows a lot of cracks on the surface, but the sampleswith nanowires show no or minimal cracks.

In various embodiments, when a certain or predetermined size ofnanoparticles (e.g., 5-20 nm) is used, the optimized weight ratio andlength of nanowires are found to be about 40:1 (NPs:NWs) and ˜40 μm,respectively. In a similar manner, when different sizes of NPs and NWsare applied, the optimum ratio and length may also be changed.

FIG. 11 shows a plot 1180 of in-situ resistance measurements for twodifferent samples of nano copper (Cu) pastes having Cu nanoparticles,and with Cu nanowires (result 1182) or without nanowires (result 1184).The samples were annealed in a reflow oven with five zones havingdifferent temperature profiles, which were 220° C., 240° C., 260° C.,280° C. and Cooling. Plot 1180 shows that the resistance of the samplewith nanowires (result 1182) drops more rapidly than the sample withoutnanowires (result 1184), and reaches the <10Ω range at the 240° C. zone,and the resistance is quite stable. The results 1182, 1184 show that theprocess temperature for the paste with nanowires may be lower than thepaste without nanowires. The results 1182, 1184 demonstrate thatnanowires help to improve the electrical properties for nano Cu pastejoining, corresponding with the results shown in FIG. 10A.

It is estimated that the global market for Flip Chip Technology by waferbumping process reached $18.9 billion in 2012. This market is estimatedto be $20.1 billion in 2013 and expected to grow to $36.5 billion in2018. Copper (Cu) pillar process, as a segment, is estimated to be $7.3billion in 2013. It is further expected to grow to nearly $24.9 billionin 2018. Various embodiments may be applied for packaging material:three dimensional integrated circuit (3D IC) joining, power electronicssoldering, high reliability bonding. It is expected that high stabilityalloy solder materials containing expensive novel metals (gold (Au),silver (Ag), platinum (Pt)) may be replaced with the material of variousembodiments, because it shows comparable electrical and mechanicalproperties and much higher reliability even for a lower price. Moreover,costs may be reduced by eliminating some procedures in the packagingprocess: formation of capping or barrier layer on a Cu bump and UnderBump Metallization (UBM) on a Cu bump or line.

While the invention has been particularly shown and described withreference to specific embodiments, it should be understood by thoseskilled in the art that various changes in form and detail may be madetherein without departing from the spirit and scope of the invention asdefined by the appended claims. The scope of the invention is thusindicated by the appended claims and all changes which come within themeaning and range of equivalency of the claims are therefore intended tobe embraced.

1. A conductive paste having a composition comprising a plurality ofconductive nanoparticles and a plurality of conductive nanowires,wherein a weight ratio of the plurality of conductive nanoparticles tothe plurality of conductive nanowires is between 10:1 and 50:1, andwherein a majority of the conductive nanowires of the plurality ofconductive nanowires has a length ranging from 20 μm to 50 μm.
 2. Theconductive paste as claimed in claim 1, wherein a majority of theconductive nanoparticles of the plurality of conductive nanoparticleshas a diameter ranging from 5 nm to 20 nm.
 3. The conductive paste asclaimed in claim 1, wherein the plurality of conductive nanowires hassubstantially uniform length.
 4. The conductive paste as claimed inclaim 1, wherein the majority of the conductive nanowires of theplurality of conductive nanowires has a diameter ranging from 100 nm to200 nm.
 5. The conductive paste as claimed in claim 1, wherein themajority of the conductive nanowires of the plurality of conductivenanowires has an aspect ratio ranging from 50 to
 500. 6. The conductivepaste as claimed in claim 1, wherein the plurality of conductivenanowires comprise a metal, and wherein the metal is selected from thegroup consisting of copper, silver, gold, alloys thereof, andcombinations thereof.
 7. The conductive paste as claimed in claim 1,wherein the plurality of conductive nanoparticles and/or the pluralityof conductive nanowires comprise copper.
 8. The conductive paste asclaimed in claim 1, wherein one or more conductive nanoparticles of theplurality of conductive nanoparticles is encapsulated with an organiclayer.
 9. The conductive paste as claimed in claim 8, wherein theorganic layer comprises an amine.
 10. A method for forming aninterconnection comprising: applying a conductive paste as claimed inclaim 1 between a first substrate portion and a second substrateportion; and fusing the plurality of conductive nanoparticles of theconductive paste to each other to interconnect the first substrateportion and the second substrate portion.
 11. The method as claimed inclaim 10, wherein fusing the plurality of conductive nanoparticles ofthe conductive paste to each other comprises heating the conductivepaste.
 12. The method as claimed in claim 11, wherein the heating occursfor a predetermined duration ranging from 6 minutes to 30 minutes. 13.The method as claimed in claim 11, wherein a predetermined peaktemperature of the heating ranges from 200° C. to 350° C.
 14. The methodas claimed in claim 13, wherein the heating at the predetermined peaktemperature occurs for a predetermined duration ranging from 90 secondsto 10 minutes.
 15. The method as claimed in claim 10, wherein the firstsubstrate portion and the second substrate portion are separate fromeach other and arranged one over the other.
 16. The method as claimed inclaim 10, further comprising forming the conductive paste comprising:providing a plurality of conductive nanoparticles; providing a pluralityof conductive nanowires; and mixing the plurality of conductivenanoparticles and the plurality of conductive nanowires in a solvent.17. The method as claimed in claim 16, wherein providing a plurality ofconductive nanowires comprises forming the plurality of conductivenanowires by an electroplating method using an anodic aluminum oxide asa template.
 18. An electrical device comprising: a first substrateportion; a second substrate portion; and a conductive member arranged tointerconnect the first substrate portion and the second substrateportion, wherein the conductive member is made of the conductive pasteas claimed in claim 1, the conductive paste processed to fuse theplurality of conductive nanoparticles of the conductive paste to eachother.
 19. The electrical device as claimed in claim 18, wherein thefirst substrate portion and the second substrate portion are separatefrom each other and arranged one over the other.